Titanium is an important metal commonly used in industry due to its desirable properties such as light mass, high strength, corrosion resistance, biocompatibility and high thermal resistivity. Thus, titanium has been identified as a material suitable for a wide variety of chemical, aerospace, and biomedical applications.
Titanium typically exists in nature as TiO2, more specifically as ilmenite (51% TiO2) and rutile (95% TiO2). Ilemenite and rutile are examples of a “titanium oxide source” material. In TiO2 the oxygen is dissolved into a Ti lattice to form an interstitial solid solution. It is difficult to remove oxygen in a Ti lattice since the thermodynamic stability of the interstitial oxygen is extremely high. Historically, the production of Ti metals from an ore containing TiO2 has been achieved through a reduction process.
There are several approaches that have been reported to reduce a Ti ore to a Ti metal. One of the oldest methods, which is still being used in industry, is the Kroll process. The Kroll process was invented by Wilhelm Kroll and is described in 1983 in U.S. Pat. No. 2,205,854 titled Method for Manufacturing Titanium and Alloys Thereof. In the Kroll Process titanium containing ores such as refined rutile or ilmenite are reduced at 1000° C. with petroleum-derived coke in a fluidized bed reactor. Next, chlorination of the mixture is carried out by introducing chlorine gas, producing titanium tetrachloride TiCl4 and other volatile chlorides. This highly volatile, corrosive intermediate product is purified and separated by continuous fractional distillation. The TiCl4 is reduced by liquid magnesium (15-20% excess) at 800-850° C. for 4 days in a stainless steel retort to ensure complete reduction according to the following formula: 2Mg(l)+TiCl4 (g)→2MgCl2(l)+Ti(s) [T=800-850° C.]. The resulting product is a metallic titanium sponge, which can be purified by removing MgCl2 through vacuum distillation. This process takes 4 days.
In a similar, and slightly older approach (Hunters process), reduction of the TiCl4 intermediate is carried out using sodium metal. Both the Kroll process and Hunter's process are costly, use high temperatures and corrosive intermediates and require long processing durations of between 4-10 days.
To overcome these drawbacks and to improve the productivity and to reduce the cost, another method, which used electrolysis was developed by Derek John Fray, Thomas William Farthing, and Zheng Chen (herein the “FFC process”). The FFC process was described in 1999 in an application titled Removal of Oxygen from Metal Oxides and Solid Solutions by Electrolysis in a Fused Salt published as WO1999064638 A1.
In the FFC process, molten calcium chloride is used as an electrolyte, TiO2 pellets are placed at the cathode and graphite is used as the anode. Elevated temperatures around 900-1000° C. are used to melt the calcium chloride since its melting point is 772° C. A voltage of 2.8-3.2 V is applied, which is lower than the decomposition voltage of CaCl2. When the voltage is applied at the cathode, oxygen in the TiO2 abstracts electrons and is converted into oxygen anions and passes through the CaCl2 electrolyte to the graphite anode forming CO/CO2 gas. In this reduction process titanium +4 is reduced to titanium 0 (i.e., metallic titanium). The pellet created in this electrolysis is then crushed and washed with HCl and consecutively with distilled water to remove the CaCl2 impurities. The resulting product is titanium metal.
Although, it was once anticipated that the FFC process would largely replace the Kroll process, there remain unresolved issues that limit its practical implementation. Some of the major drawbacks include the required use of a large amount of molten salt, slow reaction rates, the creation of undesirable intermediate products CaTiO3, Ti3O5, Ti2O3 and TiO, an impure final product and difficulties in process scalability.
In 2004, a method for creating titanium powder through calcium vapour reduction of a TiO2 preform was described in the Journal of Alloys and Compounds titled “Titanium powder production by preform reduction process (PRP).” In that method, a calciothermic reduction was performed on a TiO2 preform, which was fabricated by preparing a slurry of TiO2 powder, flux (CaCl2 or CaO), and collodion binder solution. The resulting preform was sintered at 800° C. for 1-2 h to remove binder and water before reduction. This sintered TiO2 preform was suspended over a bed of calcium shots in a sealed stainless steel reaction container. Next, the sealed reaction chamber was heated to 1000° C. where the preform was reacted with calcium vapour for 6-10 h. After cooling, the preform was dissolved in acetic acid to remove the flux and excess reductant. The resulting titanium metal was purified by rinsing with HCl, distilled water, alcohol, and acetone and then dried in vacuum. This process has several notable drawbacks including a necessarily long reaction time of 6-10 h and the undesirable formation of impurities such as CaTiO3, Ti3O5, Ti2O3 and TiO.
Magnesium vapour has been used to reduce certain metals. For example, U.S. Pat. No. 6,171,363 (the “'363 patent”) describes a method for producing Tantalum and Niobium metal powders by the reduction of their oxides with gaseous magnesium. In the process of the '363 patent, with respect to the production of tantalum powder, tantalum pentoxide was placed on a porous tantalum plate which was suspended above magnesium metal chips. The reaction was maintained in a sealed container at 1000° C. for at least 6 h while continuously purging argon. Once the product was brought to room temperature passivation of the product was done by introducing argon/oxygen mixtures, containing 2, 4, 8, 15 inches (Hg, partial pressure) of O2(g), respectively, into the furnace. Each gas mixture was in contact with powder for 30 minutes. The hold time for the last passivation with air was 60 minutes. Purification of tantalum powder from magnesium oxide was done by leaching with dilute sulfuric acid and next rinsed with high purity water to remove acid residues. The product was a free flowing tantalum, black powder.
In 2013, a process was presented in a Journal of the American Chemical Society article titled “A New, Energy-Efficient Chemical Pathway for Extracting Ti Metal from Ti Minerals” that described using magnesium hydride to produce titanium from titanium slag. In that method Ti-slag was used which contained 79.8% total TiO2 (15.8% Ti2O3 reported as TiO2), 9.1% FeO, 5.6% MgO, 2.7% SiO2, 2.2% Al2O3, 0.6% total other metal oxides. The Ti-slag was ball milled for 2 h with a eutectic mixture of 50% NaCl and MgCl2. Prior to adding the eutectic mixture, it was melted, cooled and crushed. Next MgH2 was mixed into the mixture for an hour in a laboratory tumbler. This mixture was heated in a tube furnace at 500° C. for 12-48 h in a crucible while purging hydrogen at 1 atm. The reduced product was leached in NH4Cl (0.1 M)/NaC6H7O7 (0.77 M) solution at 70° C. for 6 h, this washing step is done to remove the produced MgO. Next the product was rinsed with water and ethanol and then with NaOH (2 M) solution at 70° C. for 2 h, to remove any silicates. Next it was rinsed again and was leached with HCl (0.6 M) at 70° C. for 4 h, to remove the remaining metal oxides such as Fe. The produced TiH2 was rinsed again and was dried in a rotary drying kiln. The TiH2 powder was dehydrogenated at 400° C. in an argon atmosphere to produce Ti metal.
Each of the above-described methods presents one or more undesirable drawbacks, including but not limited to, the creation of undesirable impurities, the use of high temperatures, long reaction times, scaling constraints, and the formation of corrosive, dangerous intermediaries.